From: Andrew Morton <akpm@linux-foundation.org>
To: Pedro Falcato <pfalcato@suse.de>
Cc: "Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>,
"David Hildenbrand" <david@kernel.org>, "Zi Yan" <ziy@nvidia.com>,
"Baolin Wang" <baolin.wang@linux.alibaba.com>,
"Liam R. Howlett" <liam@infradead.org>,
"Nico Pache" <nico.pache@linux.dev>,
"Ryan Roberts" <ryan.roberts@arm.com>,
"Dev Jain" <dev.jain@arm.com>, "Barry Song" <baohua@kernel.org>,
"Lance Yang" <lance.yang@linux.dev>,
"Usama Arif" <usama.arif@linux.dev>,
"Kiryl Shutsemau" <kas@kernel.org>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
stable@vger.kernel.org
Subject: Re: [PATCH] mm/khugepaged: reject shmem collapse ranges beyond i_size
Date: Thu, 12 Nov 2026 14:50:36 -0800 [thread overview]
Message-ID: <20261112145036.97f74a828755d5b9178a3e3a@linux-foundation.org> (raw)
In-Reply-To: <avYBFPhB46XWAG4A@pedro-suse.tail5790ac.ts.net>
On Thu, 12 Nov 2026 18:11:35 +0000 Pedro Falcato <pfalcato@suse.de> wrote:
> On Thu, Nov 12, 2026 at 02:23:35PM +0000, Jérémy Jean wrote:
> > MADV_COLLAPSE lets collapse_file() treat absent shmem pages as holes
> > filled with zeros. It does not check i_size, which allows one present
> > page to become a huge mapping that covers the whole collapse range,
> > including pages beyond EOF. This makes those pages writable. A later
> > file extension can expose their contents after F_SEAL_WRITE.
> >
> > Hold inode i_rwsem in read mode while collapsing shmem so i_size
> > cannot change. Reject a collapse if its page range extends past
> > i_size rounded up to PAGE_SIZE. Keep i_size_read() outside the
> > page-cache XArray lock because its 32-bit SMP reader is not IRQ
> > safe.
>
> I don't think this is an issue. shmem/tmpfs purposefully maps with large pages
> across i_size, and collapsing across i_size is a feature, not a bug.
>
hoo boy, this again, See
https://lore-kernel.gnuweeb.org/linux-mm/nwkpojnictbg6jicccpxyfi63ol4tmfajrlx24u5xxoxx7lv5f@jkw3o7j6tlfs/T/
and enjoy.
It's not good that this sequence permits non-zero data to be exposed in
areas which ftruncate() created - the ftruncate() contract says it
doesn't do that.
Jérémy , you haven't explained why this matters. Is there some
userspace code which is encountering problems because of this?
Why did you propose cc:stable backporting?
Thanks.
prev parent reply other threads:[~2026-11-12 22:50 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-12 14:23 Jérémy Jean
2026-11-12 18:11 ` Pedro Falcato
2026-11-12 22:50 ` Andrew Morton [this message]
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